The exemplary embodiment relates to image rendering and finds particular application in connection with a content-based profile selection method.
Color image rendering is important for accurate and consistent color reproduction. Color device profiles are commonly employed to transform color image data formatted for one device color space to color image data formatted for another device color space. For example, an International Color Consortium (ICC) workflow calls for converting color image data formatted in red-green-blue (RGB) color space for a color monitor to a device independent color space called profile connection space (PCS), and then into color image data formatted in CMYK space for a color printer.
One-dimensional Tone Reproduction Curves (TRCs) are widely used in digital imaging as a means for compensating for non-linearities introduced by an individual imaging device. In general, the same TRC is used for each pixel in an image. Conventionally, one TRC is used for each of the colorants, i.e., four TRCs in the case of a CMYK process color marking device, a single TRC for a monochrome (single color) device. The TRCs can be periodically refined by printing calibration patches on the imaging device, comparing the measured color of each of the calibration patches with that of respective reference colors, and modifying the TRCs to achieve a closer correlation between the printed calibration patches and reference colors. More recently, multidimensional look up tables (LUTs) replace the single TRCs which are developed and updated frequently, allowing colorant interactions to be taken into consideration.
Recently developed control based advanced profiling approaches build a single destination profile per each halftone screen (such as for 150 dpi (dots per inch), 175 dpi, 200 dpi, etc. screens) that maps the RGB or CMYK images to device CMYK space for rendering on a process color marking engine. These approaches use the ICC image path to apply the transformation to each pixel and process the images prior to rendering in the print engine. The profile transformation is a multi-dimensional look up table (LUT) with finite number of nodes in which CMYK values are pre-computed. Colors which are not in the LUT nodes are transformed to CMYK using interpolation.
Using one profile for the whole image creates limitations for mapping out-of-gamut colors, inversion, gray component replacement (GCR), filtering etc, and hence the output may not result in the best color consistency and reproduction in one or across multiple engines. In some existing methods, source profiles are embedded inside the incoming image. However, the profiles cannot be independently tuned for different parts of the image or adjusted depending on the marking engine's reproducible gamut.